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Dictionary.com claims that the plural of 'apparatus' is 'apparatuses'. Surely that can't be right... isn't it 'apparati'?

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Try it out and see. It certainly comes from a Latin word, but did it bring along its Latin plural form? Many don't. –  John Lawler Jan 25 at 18:28
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@JohnLawler: Should I check its passport into English-land then? –  einpoklum Jan 25 at 18:30
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"Apparatus" has Latin plural "apparatus" (not "apparati"--my Latin teachers would be appalled). English can either accommodate the Latin plural "apparatus" or use the English plural "apparatuses" of the anglicized word. Both are correct. –  MPW Jan 25 at 23:50
    
Definitely never octopi - the original word is Greek but is octopous; so octopus is always an English word taking an English plural. Antipodes is a Greek plural, but oddly one never sees thee singular, although when people in Europe say that I'm from the antipodes I point out that so are they - I'm from the southern antipous, they're from the northern. An otherwise useful website referred to in this discussion claims that datum is never used in English; an engineer will quickly disabuse you of that misconception; certainly I prefer to treat data as plural but wouldn't say it was wrong to do ot –  user72194 Apr 16 at 1:51
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4 Answers

up vote 49 down vote accepted

The Latin plural of the noun apparatus is actually apparatus (the vowel lengthens, but that's not reflected in the spelling). This is because it's fourth declension. In the 18th and 19th centuries, when most educated English speakers had studied Latin, apparatus was sometimes used as the plural; I believe this usage is quite rare today.

With status, apparatus, and other Latin nouns of the fourth declension ending in -us, the English plural adds an -es. The Latin nouns which pluralize by turning an -us into an -i, like radius, fungus, alumnus, are all second declension. For some Latin nouns of the second declension, like campus, the Latin plural has been lost, and the English plural adds an -es. But campi would be correct in Latin, while apparati is incorrect in both English and Latin.

You don't actually hear the English plural apparatuses that often, because apparatus is treated as a semi-uncountable noun: one apparatus, two pieces of apparatus. (See Google Ngrams.) I assume this usage developed because people didn't know whether to use apparatus or apparatuses for the plural, and figured out a way of avoiding the issue altogether.

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In technical and scientific work dealing with this and that corpus, Latin’s third declension plural of corpora came right along with it. O tempora, o mores! (Ciceronian citation because that’s only other third declension noun I could quickly (tempus fugit) think of that people might generally recognize.) –  tchrist Jan 25 at 19:19
    
How does the vowel extension reflect? Also, where can I read about determining which declension a noun is? –  einpoklum Jan 25 at 22:29
    
Latin declensions are found by looking at the genitive, given as the second word form in a Latin dictionary, and/or are simply memorized. Long vowels in Latin may be written under a macron, eg. status → statūs, but this is inconsistent usage across time, place, et al. and is often omitted. –  BRPocock Jan 25 at 22:46
    
I agree with Peter Schor: "The Latin plural of the noun apparatus is actually apparatūs (the vowel lengthens, but that's not reflected in the spelling)." –  Laura Morland Jan 26 at 2:31
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What Peter said. If you really have to make a plural of the English word, you could use apparatus, which is given as a plural in the OED. But the OED also gives apparatuses, which seems a much better bet.

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And to further complicate things, apparatus is usually a mass noun, so if you dealing with several pieces of apparatus, according to the OED, all of the following are grammatical: the apparatuses are ready, the apparatus are ready, and the apparatus is ready. I'd use the last one. –  Peter Shor Jan 25 at 19:01
    
I think I would too. –  Barrie England Jan 25 at 19:30
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As @JohnLawler said, not all Latin words keep their Latin plurals. Consider:

  • virus/viruses (not virii, see link)
  • fetus/fetuses
  • victor/victors
  • vector/vectors
  • minus/minuses
  • onus/onuses
  • sinus/sinuses
  • status/statuses (yes, statuses)
  • stadium/stadiums (Greek word actually but came to English through Latin and (almost) nobody says stadia)

And the list goes on. I have no Latin so I don't know how many of these don't keep their plural because of the rule mentioned in @PeterShor's answer but you can see that many Latin words have normal English plurals. In other words, trust the dictionary, it tends to know more than we do.

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"nobody says stadia". Well, in Dutch they do. Using the same Latin root (although interestingly, the Dutch use 'stadion' - the Greek word - for the sports arena, and 'stadium' for 'era / epoch'. –  Floris Jan 26 at 3:00
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@Floris well, yes, OK, in other languages sure. I was talking about English. We say stadia (στάδια) in Greek as well. –  terdon Jan 26 at 15:41
    
"Nobody says stadia". It's not uncommon in the UK: see, for example, the UK Parliament, the BBC, the Daily Telegraph. –  David Richerby Jan 26 at 18:03
    
@DavidRicherby well yes, that'swhy I mentioned that nobody says it. I know it exists, it's just very rarely used in normal conversation which is why I said that nobody says it. Some may of course write it but, fair enough, answer edited. –  terdon Jan 26 at 18:28
    
@terdon I don't think this has anything to do with written versus spoken English. Anyone who writes "stadia" is going to say it, too; if it's true that the word is rarely used in spoken English, it's because people rarely need to talk about more than one stadium, not because people write "stadia" but say "stadiums". –  David Richerby Jan 26 at 23:30
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To write "apparati" is an example of a hypercorrection:

In linguistics or usage, hypercorrection is a non-standard usage that results from the over-application of a perceived rule of grammar or a usage prescription. A speaker or writer who produces a hypercorrection generally believes that the form is correct through misunderstanding of these rules, often combined with a desire to appear formal or educated.

It's the pedant's own-goal, basically. I've made many. Using "whom" as a subject pronoun is a classic.

Some more examples similar to the question are:

  • octopi instead of octopuses
  • scenaria or scenarii instead of scenarios
  • penii instead of penises

Although in some cases (see "octopi" vs "octopuses" on Ngrams) the hypercorrected form may be common enough to be accepted as standard usage.

In the case of "syllabus", for instance, an analysis of the Latin shows "syllabi" to be a hypercorrection. However Ngrams shows it to be more widely used than the supposedly more correct "syllabuses".

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